1. Field of the Invention
The present invention relates to a decryption key management scheme for managing a decryption key for decrypting an encrypted software in a software distribution system.
2. Description of the Background Art
Conventionally, a software to be used by a computer is individually stored in a recording medium such as floppy disks and usually sold at a retail store in a form of a software package which contains the recording medium along with a manual and other necessary items.
In a case of such a software package, the licensing to give a permission to use the software is usually made in a form which is inseparable from the sale of the software package. For instance, there are cases in which a document describing that a contract to use the software is established as soon as the software package is opened is distributed along with the software package itself, or cases in which a user is required to sign a contract to use the software with the software vender by using a user registration form enclosed in the software package.
In contrast, recently, there are propositions for a new software distribution scheme aimed at realization of a freer distribution style and a tighter software right owner protection. As a representative example of such a new software distribution scheme, the so called superdistribution scheme is well known (see: R. Mori and M. Kawahara, "Superdistribution: The Concept and the Architecture", The Transactions of the IEICE, Vol. E73, No. 7, pp. 1133-1146, July 1990; and U.S. Pat. No. 5,103,392).
The superdistribution scheme utilizes a software structure formed by an encrypted software content and a header portion having a function to decrypt the encrypted software content. In addition, two concepts called "S-credit" and "Software Usage Monitor" are utilized to realize the licensing between a user and a software right owner and the management of software usage charges.
"S-credit" is realized in a form of a memory card such as an IC card which stores information as to quantitatively how much software usage is to be permitted to a user. A user who wishes to use the software distributed by the superdistribution first pays an appropriate fee at a service center of the superdistribution system, and receives the "S-credit" which registers the information corresponding to the paid fee.
"Software Usage Monitor" is a program which recognizes a balance of the user's contract from a value of the information registered in the "S-credit", and permits or prohibits the execution of the software according to the recognized balance of the user's contract. This program permits the execution of the software after confirming the availability of the "S-credit" indicating a balance greater than a certain value on the computer, and decreases the value of the information registered in the "S-credit" (indicating the balance) as much as a fee to be charged for this software.
However, in such a method for managing the software usage charges using "S-credit", there is a need to provide a protection against illegal copying of "S-credit" or illegal rewriting of data of "S-credit", and to this end, it has been necessary to use a specialized hardware mechanism for limiting read/write with respect to "S-credit", for example.
For this reason, recently, there is a proposition for a new software distribution system in which the encrypted software content is distributed to users for free or for a very small charge, and a key for decrypting the encrypted software content is issued to a user who wishes to use this software in exchange for the payment of the software usage charge.
In this software distribution system, the software vender can save costs required for production and distribution of individual software packages, and in addition, an effective means for advertising the software can be provided by packaging a non-encrypted demonstration version of this software for demonstrating the encrypted software content in a medium such as CD-ROM which stores the encrypted software content. From a viewpoint of a user, this feature can provide an advantage in that there is a chance to try and see if the software is worth purchasing before actually purchasing the software.
The user receives the decryption key in exchange for the payment of the software usage charge for a desired software, and installs this software into a hard disk device of his own computer by decrypting the encrypted software content using the received decryption key.
In this manner, the software distribution system utilizing the decryption key can manage the software usage charge in a form of the exchange of the decryption key, so that there is no need for "S-credit" and a specialized hardware for managing "S-credit" required by the superdistribution scheme, and therefore it is possible to realize a freer software distribution style.
Now, considering a usual manner by which a user uses a computer, it often becomes necessary to delete the already installed software once in order to increase a usable capacity of the hard disk device, and then re-install the same software back to the hard disk device later on when a need to use this software arises. In a case of using the usual non-encrypted, packaged software, the user still possesses the recording medium contained in the software package itself, so that the deletion of the installed software on the hard disk device causes no problem, because it is possible to re-install the software from the recording medium when this software becomes necessary later on.
In contrast, in a case of the above described software distribution system utilizing the decryption key, the decrypted software which was installed on the hard disk device by using the decryption key is the only executable software available for a user. Consequently, when the user loses the decryption key which was acquired from the software vender in exchange for the software usage charge and wishes to re-install the same software, it is necessary to acquire the decryption key by paying the software usage charge for the same software again. This fact can lead to a lower reliability of the software distribution system utilizing the decryption key, which in turn can present a hindrance to a spread of this software distribution system utilizing the decryption key.